In the study of electronic materials and processes for fabricating such materials into an electronic structure, a specimen of the electronic structure is frequently used for microscopic examination for purposes of failure analysis and device validation. For instance, a specimen of an electronic structure such as a silicon wafer is frequently analyzed in scanning electron microscope (SEM) and transmission electron microscope (TEM) to study a specific characteristic feature in the wafer. Such characteristic feature may include the circuit fabricated and any defects formed during the fabrication process. An electron microscope is one of the most useful equipment for analyzing the microscopic structure of semiconductor devices.
In preparing specimens of an electronic structure for electron microscopic examination, various polishing and milling processes can be used to section the structure until a specific characteristic feature is exposed.
As device dimensions are continuously reduced to the sub-half-micron level, the techniques for preparing specimens for study in an electron microscope have become more important. The conventional methods for studying structures by an optical microscope cannot be used to study features in a modern electronic structure due to the unacceptable resolution of an optical microscope.
In the focused ion beam (FIB) technique, focused ion beam is used to either locally deposit or remove materials. Typical ion beams have a focused spot size of smaller than 100 nm when produced by a high intensity source. Sources of such high intensity ions can be either liquid metal ion sources or gas field ion sources. Both of these sources have a needle type form that relies on field ionization or evaporation to produce the ion beam. After the ion beam is produced, it is deflected in a high vacuum and directed to a desired surface area. The focused ion beams can be suitably used in the semiconductor processing industry in a cutting or attaching method to perform a circuit repair, a mask repair or a micromachining process. A cutting process is normally performed by locally sputtering a surface with a focused ion beam.
In an ion beam milling process, a material is selectively etched by a beam of ions such as Ga+ focused to a sub-micron diameter; the technique is often referred to as focused ion beam etching or milling. FIB milling is a very useful technique for restructuring a pattern on a mask or an integrated circuit, and for diagnostic cross-sectioning of microstructures. In a typical FIB etching process, a beam of ions such as Ga+ is incident onto a surface to be etched and the beam is deflected to produce a desirable pattern. The focused ion beam can be used to bombard a specimen surface such that a cavity is formed on the surface of an electronic structure to reveal a characteristic feature of the structure for electron microscopic examination. The FIB technique utilizes a primary beam of ions for removing a layer of material at a high current, and for observing the surface that was newly formed at a low current. The observation of the surface is made by detecting the secondary electrons emitted from the sample surface when the surface is bombarded by the ions. A detector is used to receive the secondary electrons emitted from the surface to form an image. Even though the FIB method can not produce an image of a high resolution like that obtainable in a SEM/TEM, the FIB method can be used to sufficiently identify a newly formed cross-sectional surface which may contain the characteristic feature to be examined. The capability of the FIB technique for making observations down to a resolution of 5˜10 nm enables the cutting of a precise plane in an electronic structure such that the electronic structure may be later examined by a SEM or TEM technique at a higher resolution than that capable with FIB.
Although TEM techniques can provide a higher resolution image and a more detailed description of the internal structure of a electronic structure than is available using SEM techniques, they are only effective for electron transparent samples. Thus it is a basic requirement for TEM samples that the sample must be thin enough to be penetrated by the electron beam and thin enough to avoid multiple scattering, which causes image blurring. Nonetheless, it is recognized in the art that thin samples extracted from wafers may be brittle, and subject to fracture or crumbling. Furthermore, the fragile nature of thin extracted samples means that processes for extracting thin samples are difficult to automate, thus hindering efforts to automate these processes. There is an ongoing need for reliable and automated techniques for the obtaining and imaging of TEM samples to make TEM sampling a viable part of semiconductor analysis and manufacturing.
An additional technique for conducting electron transparency analysis with nanometer level spatial resolution is scanning transmission electron microscopy (STEM). The TEM is an apparatus in which an electron beam is irradiated onto a sample, and the transmitted electron beam is magnified using a lens. On the other hand, the STEM is an apparatus in which an electron beam is focused onto a micro-area, and a two-dimensional image is obtained by measuring intensities of the transmitted electron beam while the electron beam is being scanned on the sample. US 20030127595, incorporated herein by reference, discloses methods and apparatus for scanning transmission electron microscopy.
Enabled by automated, multicolumn tools combining SEM and FIB in a single device, automated techniques for the obtaining and imaging of SEM samples are already well known and are employed in the automated reviewing of defects and for process monitoring. Examples of commercially available models of such multicolor tools include SEMVision™ G2 FIB (Applied Materials, Santa Clara, Calif. and the DualBeam™ (FEI Company, Hillsboro, Oreg.). It is noted that the SEF is also used in process control.
Below is enumerated a list of United State Patents, published United States Patent applications that disclose potentially relevant background material. Each of the following United States Patents and published United States Patent application are incorporated herein by reference in their entirety:
U.S. Pat. No. 6,194,720 of Li et al., titled “Preparation of Transmission Electron Microscope Samples”;
U.S. Pat. No. 6,670,610 of one of the present inventors and coworkers, titled “System and Method for Directing a Miller”;
U.S. Pat. No. 6,700,121 of Kelly et al., titled “Methods of Sampling Specimens for Microanalysis”;
U.S. published patent application 2001/0044156 of Kelly et al., titled “Methods of Sampling Specimens for Microanalysis”;
U.S. published patent application 2001/0045511 of Moore et al, titled “Method For Sample Separation and Lift-Out”;
U.S. published patent application 2002/0000522 of Alani, titled “Ion Beam Milling System and Method for Electron Microscopy Specimen Preparation”;
U.S. published patent application 2002/0121614 of Moore et al, titled “Total Release Method for Sample Extraction from a Charged-Particle Instrument”;
U.S. published patent application 2004/0245466 of Robinson et al, titled “Transmission electron microscope sample preparation”;
U.S. published patent application 2004/0129897 of Adachi et al., titled “Sample manufacturing apparatus”;
U.S. published patent application 2004/0164242 of Grunewald titled “Sample preparation for transmission electron microscopy”;
U.S. published patent application 2004/0246465 of Iwasaki et al. titled “Micro-sample pick-up apparatus and micro-sample pick-up method”;
U.S. published patent application 2004/0178355 of Rasmussen titled “Sample Manipulation System”.
Presently, in most semiconductor manufacturing facilities, electron transparency samples of semiconductor wafers for microscopy analysis are obtained on site, and subsequently shipped to an electron microscopy laboratory for electron transparency analysis.
Time delayed process monitoring and defects identified in a manufacturing process only after a certain time delay could be expensive for a semiconductor manufacturer, and thus it is desirable that defective manufacturing processes be identified as soon as possible and preferably on site.
There is an ongoing need for methods and systems for extracting samples from wafers for electron transparency microanalysis. Preferably, such methods would be implementable in a single tool. More preferably, such methods would be automated within a single tool, in order to facilitate their integration into the semiconductor manufacturing process. Preferably, such methods would allow for extraction of a sample without boring a hole in the wafer, splitting the wafer, or otherwise rendering the semiconductor wafer unusable after sample extraction.
Furthermore, it is recognized that in the context of defect analysis and process control analysis, it is often necessary to image a cross section surface of a sample of a wafer. Thus, there is an ongoing need for techniques for microanalyzing cross sections of samples that contain characteristic features.